This invention relates to an amorphous non-glassy ceramic composition that may be prepared by curing, calcining and/or pyrolyzing a precursor material comprising silicon, a Group III metal, a Group IVA metal, and/or a Group IVB metal. In particular, the composition is not irreversibly adsorptive for components or fluids that come into contact with it thereby allowing it to be useful as a passivation coating or film for any underlying substrate where it functions as a barrier against adsorption of components of the fluid to the underlying surface of the substrate. Further, the amorphous non-glassy ceramic composition functions as a highly useful matrix for a particulate adsorptive material, and/or as a film for adhering such adsorptive material to an underlying surface. Still further, the precursor material may comprise a polysilazane, polysiloxane, and particulate adsorptive material forming a fluid-permeable mass useful, for example, as an improved adsorptive bed in pipette tips.
There has been a long-identified need in scientific analytical techniques, such as chromatographic applications, to have the ability to coat a substrate thereby causing the substrate to be non-reactive with respect to a target analyte that comes into contact with the substrate. In particular, with respect to chromatographic applications, a coating useful to provide a barrier against adsorption of components in a fluid to the surface of a vessel, conduit, or device which is in contact with the fluid. Interactions between the target analyte and the surfaces of a vessel, conduit, and/or device may affect analytical results. This affect may be more pronounced in SPME applications where only a very minute amount of analyte is adsorbed. U.S. Pat. No. 5,192,406 discloses certain surface-deactivation techniques using polymeric silyl hydrides, siloxanes, silazanes and silicone polymers to deactivate glass or fused-silica CZE capillary columns without eliminating electro-osmotic flow. Further, deactivation of capillary columns for gas chromatography is also disclosed. However, no discussion is given to parts of the chromatographic apparatus other than the glass or fused silica columns.
Recently, the formation of silicon containing passivation films on various substrates has been dominated by techniques such as Chemical Vapor Deposition (CVD), Plasma CVD or wet chemical methods, including Sol Gel. These techniques can be effective, but suffer from several drawbacks requiring further improvement.
Silicon containing film forming processes utilizing CVD (e.g., U.S. Pat. No. 6,511,760 and U.S. Pat. No. 6,444,326) often suffer in some or all of the following areas: (1) contamination of the apparatus and substrate caused by formation of silicon particles in the gas phase reaction, thereby reducing production yields and/or requiring post-coating clean-up; (2) difficulty in obtaining a uniform film on uneven surfaces and/or presence of undesirable substances such as oxides in the film, caused by the gaseous nature of the raw materials; (3) low productivity caused by low film formation speeds; (4) necessity of complex and expensive equipment, such as high frequency generators and vacuum equipment; and (5) high reactivity and toxic nature of the gaseous raw materials, such as gaseous silicon hydride, requiring appropriate handling procedures and safety equipment to provide airtight conditions.
Researchers have attempted to produce passivating films from liquid silicon hydride containing raw materials with limited results. JP-A-29661 recites a process for forming a silicon-based thin film by liquefying a gaseous raw material on a cooled substrate and subsequently reacting it with active atomic hydrogen. This process involves complex equipment and is very difficult to control film thickness.
Low molecular weight liquid silicon hydride as a film-forming precursor is disclosed by JP-A-7-267621. However, the process recited is problematic due to the handling of unstable material and wettability problems associated with substrates of large surface area.
A solid silicon hydride polymer precursor was recited by GB-2077710A. However, the material is difficult to use due to poor solubility in common solvents.
Further, U.S. Pat. No. 6,503,570 discloses the synthesis of silylcyclopentasilane as a liquid film-forming precursor. This material is decomposed at temperatures below 500° C. and is easily dissolved in common organic solvents such as toluene, hexane, THF and acetone. Limitations of this approach include the complicated and costly synthetic procedure and the instability of the material in air.
U.S. Pat. No. 5,853,808 discloses the decomposition of chloroethylsilsesquioxane into thin ceramic films. Rearrangement reactions are generally conducted under intense UV light with the evolution of highly corrosive hydrochloric acid. Additionally, the high cost of production further limits this family of materials.
Sol Gel technology has been widely used in the formation of silica containing films and binder applications for small particles. Generally, the procedure involves the acid hydrolysis of metal alkoxides to form liquid sol solutions used in the above-mentioned applications. This liquid precursor technique suffers from limitations such as poor solvent compatibility and substrate wettability. Further, uneven coating and pinhole formation can be major problems along with the inability to create stable suspensions of particulate matter. Additionally, Sol solutions can be very unstable resulting in premature gelation and short shelf life. U.S. Pat. No. 4,277,525 describes a complicated method to eliminate the shortcomings of the technique. Generally, an alkoxysilane is mixed with a carboxylic acid or anhydride with a pK larger than 4. A third reactant, a monovalent or divalent alcohol such as methanol, ethanol or ethylene glycol, is added. A reaction accelerator is described as a different carboxylic acid with a pK not exceeding 4. Sol formation proceeds over several hours. Precautions are necessary so the exothermic reaction does not raise the temperature of the sol over 50° C. or gelation may occur. Along with the complexity of the Sol preparation, the large amounts of residual carboxylic acid may create wetting problems on some substrates.
To facilitate the handling of adsorbent particles, it may be helpful to agglomerate the particles into shaped forms, beads or the deposition of these particles on a substrate. A stable adherent coating of particles that resists delamination from the substrate in use may be advantageous for a variety of reasons, including (1) improvement of surface area to weight ratio; (2) reduction in the total amount of adsorbent required (3); protection of the underlying substrate from aggressive environments; and (4) the geometry of the substrate may be required to provide strength or form to the adsorbent system. A binding material is needed that allows for easy suspension of particulates, adequate adhesive properties to a variety of substrates, high stability and inertness in both liquid precursor and ceramic form, low interference with adsorbent porosity.
U.S. Pat. No. 5,325,916 and U.S. Pat. No. 6,143,057 both describe a wide variety of binding materials used in the creation of adsorbent structures composed of fine zeolite materials. Suitable binders include, macroporous clays, silicas, aluminas, metal oxides and mixtures thereof. These types of binding materials may be limited to more durable adsorbents such as zeolites or silicas and may not provide the inertness required for the subsequent desorption of many organic molecules.
Further, U.S. Pat. No. 5,599,445 discloses the use of a siloxane polymer adhesive as a binding material for various adsorbents including alumina, silica, organic polymer adsorbents and zeolites. Adsorbent films may be prepared and used as chromatographic stationary phases for volatile organic and permanent gas separation. The siloxane material may be used successfully, but adversely effects pore volume of the adsorbents. Additionally, these polymeric type materials have limited thermal stability resulting in unwanted volatile by products upon decomposition.
The current commercial pipette tips exhibit fracturing of the beds during tip usage, and therefore lack sufficient capacity. Also, several commercially available tips possess coatings only on the tip walls and therefore have limited capacity with analyte recovery significantly reduced.
The present invention seeks to reduce or eliminate the limitations of known compositions and methods due to the chemical properties of the macromolecules and decreased production costs allowing for use in a variety of chromatographic systems and applications.